In general, a particle beam therapy system is provided with a beam generation apparatus that generates a charged particle beam, an accelerator that is connected with the beam generation apparatus and accelerates a generated charged particle beam, a beam transport system that transports a charged particle beam that is accelerated by the accelerator so as to gain predetermined energy and then emitted, and a particle beam irradiation apparatus, disposed at the downstream side of the beam transport system, for irradiating a charged particle beam onto an irradiation subject. Particle beam irradiation apparatuses are roughly divided into apparatuses utilizing a broad irradiation method in which a charged particle beam is enlarged in a dispersion manner by a scatterer, and the shape of the enlarged charged particle beam is made to coincide with the shape of an irradiation subject in order to form an irradiation field; and apparatuses utilizing a scanning irradiation method (the spot-scanning method, the raster-scanning method, and the like) in which an irradiation field is formed by performing scanning with a thin, pencil-like beam in such a way that the scanning area coincides with the shape of an irradiation subject.
In the broad irradiation method, an irradiation field that coincides with the shape of a diseased site is formed by use of a collimator or a bolus. The broad irradiation method is a most universally utilized and superior irradiation method where an irradiation field that coincides with the shape of a diseased site is formed so as to prevent unnecessary irradiation onto a normal tissue. However, it is required to create a bolus for each patient or to change the shape of a collimator in accordance with a diseased site.
In contrast, the scanning irradiation method is a high-flexibility irradiation method where, for example, neither collimator nor bolus is required. However, because these components for preventing irradiation onto not a diseased site but a normal tissue are not utilized, there is required a positional accuracy of beam irradiation that is the same as or higher than that of the broad irradiation method.
Patent Document 1 discloses a beam position monitor, for a particle beam therapy system, that has a purpose of solving the problem that in a raster-scanning irradiation method in which when the irradiation position is changed, the charged particle beam is kept to irradiate, the accuracy of beam position measurement is deteriorated mainly by the fact that the electric charges collected during the scanning of the charged particle beam and the electric charges collected when the scanning has been completed cannot accurately be distinguished from each other. The beam position monitor according to Patent Document 1 is provided with a collection electrode (corresponding to a sensor unit of the position monitor) for collecting collection charges produced by ionization of the charged particle beam, and a signal processing circuit that performs a beam position calculation for determining the beam position by utilizing collection charges. The signal processing circuit is provided with an I/V converter that generates a voltage signal obtained by I/V-converting the current output from the collection electrode; a digital signal generation circuit that generates a digital signal related to the collection charges when the voltage signal is inputted thereto; a timing signal transmission/reception unit that receives a signal, as a timing signal, that is generated at a time when the charged particle beam, which is scanned from a scanning-stop irradiation point (corresponding to an irradiation spot in a spot scanning irradiation method) to the next scanning-stop irradiation point, is in the non-scanning state (in which the charged particle beam is stopped at the scanning-stop irradiation point); and a beam position calculation unit that calculates a beam position by use of a digital signal related to collection charges when a digital signal, related to the collection charges, generated by the digital signal generation circuit at the timing when the timing signal transmission/reception unit receives the timing signal, is inputted thereto.